The present invention relates to a process for preparing hollow aluminum extrudates for use in a vacuum, and more particularly to a process for preparing hollow aluminum extrudates for use in a high vacuum, such as particle accelerating pipes for use in synchrotrons and like accelerators.
The term "aluminum" as used herein includes pure aluminum and alloys thereof.
Although particle accelerating pipes of the type mentioned were primarily made of stainless steel, aluminum has recently been found suited to this use and introduced into use because as compared with stainless steel, aluminum is less likely to induce radioactivity, more rapidly attenuates the radioactivity induced, is higher in heat and electric conductivities, has a surface which is lower in outgassing rate, is more lightweight and has higher workability.
The interior of the particle accelerating pipe must be maintained on a high vacuum for passing particles therethrough at a high speed. Accordingly, what matters is how to evacuate the pipe to a high vacuum. In order to maintain the interior of the particle accelerating pipe in a high vacuum, it is important to diminish the release of gases from the inner wall of the pipe as produced. For diminishing the release of gases from the inner surface of particle accelerating pipes of aluminum, it has been found effective to form a compact and thin coating on the surface.
The compact thin coating is effective for diminishing the release of gases for the following reason. As is well known, aluminum is a metal which is very susceptible to oxidation, such that an oxide coating is formed on the surface merely when it is brought into contact with a very small amount of oxygen. On the inner surface of an aluminum pipe extruded by the usual process, a hydrated oxide coating, such as boehmite or bialite film, is formed upon extrusion by contact with the watercontaining atmospheric air. Moreover, since the pipe being extruded is exposed to a high temperature, the coating forming reaction is accelerated to give a large thickness to the coating. Unlike the aluminum oxide coating which is formed in the absence of water, the hydrated oxide coating has an exceedingly rough texture, is porous and has pores of intricate form. The coating absorbs a large quantity of water because it is thick. After the extrusion, moreover, the coating, which is not compact, absorbs from the atmosphere substances which lower the degree of vacuum, such as water, hydrocarbons, carbon dioxide and carbon monoxide. Such vacuum reducing substances are still present in small quantities when the pipe is cleaned by an electric discharge in hydrogen gas, argon gas, oxygen gas or the like, when it is degassed with heating or when it is evacuated, so that these substances are adsorbed by the coating similarly and become difficult to remove even by evacuation. Consequently, these substances present difficulties in giving a higher vacuum to the particle accelerating pipe. To impart enhanced mechanical strength to the extruded aluminum pipe, the pipe is heated to a high temperature and then cooled in water and air for hardening. During this process, the hydrated oxide coating formed during the extrusion further grows, while the vacuum reducing substances already adsorbed become occluded by the coating. The compact thin coating is exceedingly smaller than the thick hydrated oxide coating of rough texture in the amounts of vacuum reducing substances adsorbed or occluded. Even if adsorbed or occluded, these substances are readily removable by a degassing treatment. Consequently, the amounts of such substances to be released into the pipe are greatly decreased.
A process is already known for producing a hollow aluminum extrudate formed with a compact thin oxide coating on its inner surface for use in a vacuum (see U.S. Pat. No. 4,578,973). This process is characterized in that an oxygen-containing inert gas serving as impurities is supplied to the hollow portion of the shaped material being extruded to form the compact thin oxide coating on the inner surface of the extrudate with the oxygen contained in the inert gas.
However, the process has the problem of a high production cost because the inert gas is expensive.